In a mobile communication system typified by a cellular communication system, or a wireless LAN (Local Area Network) system, a random access field is provided in a transmission field. This random access field is provided in an uplink transmission field when a terminal station (hereinafter referred to as “UE”) initially makes a connection request to a base station (hereinafter referred to as “BS”), or when a BS or the like makes a new resource allocation request in a centralized management system that allocates a UE transmission time and transmission band. A base station may also be called an access point or Node B.
With a random access burst (hereinafter referred to as “RA burst”) transmitted in a random access field (hereinafter referred to as “RA slot”), unlike other scheduled channels, a reception error and retransmission occur due to a signature sequence collision (transmission of an identical signature sequence using the same RA slot by a plurality of UE's) or due to interference between signature sequences. When an RA burst collision or reception error occurs, the processing delay of RA burst uplink transmission timing synchronization acquisition and BS connection request processing increases. Consequently, there is a demand for a reduction in the signature sequence collision rate and an improvement in signature sequence detection performances.
In the mobile communication system described in Non-Patent Document 1, as an RA burst preamble (hereinafter referred to as “RA preamble”) sequence, an RA preamble sequence (or signature sequence) that uses a Zadoff-Chu sequence (hereinafter referred to as “ZC sequence”) or GCL sequence (Non-Patent Document 2) having a low auto-correlation characteristic and inter-sequence cross-correlation characteristic is investigated. Also, the use of a ZC-ZCZ (Zadoff-Chu Zero Correlation Zone) sequence generated by performing a cyclic shift of a ZC sequence is investigated.
With a ZC sequence and GCL sequence, an auto-correlation characteristic is optimum when its sequence number r and sequence length N satisfy a relatively prime (coprime) relationship. Also, with regard to a cross-correlation characteristic between two sequences, if the sequence numbers are designated r1 and r2 respectively, the cross-correlation value is constant at √N when the absolute value of the difference between r1 and r2 and sequence length N satisfy a relatively prime relationship. Therefore, when sequence length N is a prime number, a set of sequences for which an auto-correlation characteristic and cross-correlation characteristic are optimum is obtained for N−1 sequences—that is, all sequences with sequence number r=1, 2, . . . , N−1.
Also, in the mobile communication system described in Non-Patent Document 1, always allocating 64 ZC-ZCZ sequences to one cell is investigated. These 64 sequences include ZC sequences with different sequence numbers and cyclic shift sequences—that is, ZC-ZCZ sequences—generated from ZC sequences having the respective sequence numbers.
The number of ZC-ZCZ sequences that can be generated from one ZC sequence depends on a cyclic shift amount between sequences. If the cyclic shift amount is designated Δ and the sequence length is designated N, the generated number of ZC-ZCZ sequences is expressed as floor(N/Δ), where floor(x) represents the largest integer that does not exceed x. To consider a time (Atime) corresponding to cyclic shift amount Δ, cyclic shift amount Δ is defined by a time range in which it is possible for an RA preamble transmitted from a UE to arrive. Specifically, cyclic shift amount Δtime is set so as to be greater than the sum of the maximum round-trip expected value (TRoundTripDelay) based on the propagation delay time between a BS and UE (TPropagationDelay) and the maximum expected value of channel multipath delay time (TDelaySpread) (Δtime>2×TPropagationDelay+TDelaySpread).
Therefore, since the propagation delay time between a BS and UE increases in proportion to the cell size (cell radius), the larger the cell size of a cell, the smaller is the number of ZC-ZCZ sequences that can be generated from one ZC sequence. Consequently, in order to allocate 64 preamble sequences to one cell, it is necessary to allocate many ZC sequences with different sequence numbers to the cell.
A BS generates a broadcast channel with sequence numbers of sequences used by a cell as allocation sequence information, and reports this to UE's present within the cell. Each UE generates an RA burst using a ZC sequence having a reported sequence number, and performs random access. A possible allocation sequence information report method is to report sequence numbers of sequences used by a cell one at a time. This method allows flexible sequence allocation since arbitrary sequence numbers are allocated to a cell.
Non-Patent Document 1: “3GPP TSG RAN; Physical Channels and Modulation (Release 8),” TS36.211V1.0.0
Non-Patent Document 2: “Generalized Chirp-Like Polyphase Sequences with Optimum Correlation Properties,” Branislav M. Popovic, IEEE Transaction on Information Theory, Vol. 38, No. 4, July 1992